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Radiations of solar origin penetrating below 85 km in the terrestrial atmosphere are: (1) X rays of λ<10 A; (2) Lyman α; and (3) wavelengths greater than 1800 A. These radiations can ionize: (1) molecular nitrogen and oxygen; (2) nitric oxide; and (3) various atoms such as sodium and calcium. Molecular oxygen and nitrogen are also ionized by cosmic rays. The negative ion to electron ratio is important below 70 km and affects the electron distribution below that altitude. It is possible to explain normal conditions of ionization by cosmic rays and Lyman α. Conditions due to solar flares must be explained by X rays. Above 85 km, the behavior of the ionization is related to the formation of the E layer.
Nicolet et al. (1960) studied this question.